Microscopic calculation of phonon spectrum of KCl and KBr

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Abstract
A microscopic calculation is performed of the phonon spectrum of the alkali halide crystals KCl and KBr in the approximation where the dielectric matrix is assumed to be of separable form. The calculations involve two adjustable microscopic parameters. Reasonable agreement with experiment is obtained. A detailed discussion is also given of the microscopic treatment of effective charges and overlap interactions in ionic crystals. It is shown how qualitative conclusions regarding the nature of the overlap forces may be drawn from a simple consideration of the nature of the valence- and conduction-band orbitals.